When nitrogen oxides in combustion exhaust gas of an internal combustion engine such as a marine engine is removed, an ammonia selecting reduction method is mainly adopted as a method therefor. The ammonia selecting reduction method is a method in which a denitration catalyst containing vanadium or titanium as a main component is used as a catalyst and ammonia is used as a reducing agent.
However, when a C fuel oil or the like is combusted in an internal combustion engine such as a marine engine, since sulfur components are contained in the C fuel oil or the like, sulfur oxides will also be generated in the combustion exhaust gas along with nitrogen oxides. For such combustion exhaust gas, when denitration is carried out using an ammonia selecting reduction method, sulfur oxides and ammonia are reacted in combustion exhaust gas to give ammonium sulfate [(NH4)2SO4], which deposits in facilities once the temperature of the combustion exhaust gas is lowered, resulting in clogging in a heat exchanger or other problems. For this reason, there has been a problem in that the temperature in the system has to be set to a temperature within the range where ammonium sulfate does not deposit (about 280° C. or higher).
On the other hand, as a reductive removal method using a reducing agent other than ammonia, for example, PTL 1 as listed below discloses a method in which an alcohol is used as a reducing agent along with a catalyst of a metal supported on a zeolite.
However, there has been a problem in that, when an alcohol is brought into contact with a catalyst of a metal supported on a zeolite, a side reaction also occurs besides the desired reaction and so-called coke (carbon) deposits on a surface of the catalyst due to a by-product produced by such a side reaction, resulting in deterioration of the denitration performance with time.
PTL 2 discloses that a deteriorated denitration performance is recovered by placing a denitration catalyst layer in two-divided exhaust gas processing channels, closing one of the exhaust gas processing channels to stop supply of the exhaust gas while continuing to process the exhaust gas in another exhaust gas processing channel, and heating in situ the denitration catalyst layer of the one exhaust processing channel in which the supply of the exhaust gas is stopped at 350 to 800° C.
PTL 3 discloses a reductive removal method of nitrogen oxides by bringing a metal aluminate catalyst containing a transition metal of the fourth period into contact with exhaust gas containing nitrogen oxides under an oxidizing atmosphere where an excess amount of oxygen is present and in the presence of alcohol.
On the other hand, PTL 4 discloses a removal method of nitrogen oxides by bringing one or more catalysts selected from a proton type zeolite, an alumina, and an alumina supporting a transition metal of the fourth period into contact with exhaust gas containing nitrogen oxides in an oxidizing atmosphere where an excess amount of oxygen and water vapor are present and in the presence of at least one of methanol and ethanol at a reaction temperature of 200 to 550° C.